Leucine, a key essential branched chain amino acid, may have a role in the regulation of protein metabolism. In addition, measurements of leucine kinetics have been used as a model to estimate whole body rates of protein turnover, synthesis, breakdown and oxidation. Currently, there is little information concerning leucine kinetics in the neonatal period, and limited knowledge of fetal leucine metabolism. These studies will determine leucine kinetics in specific human neonatal populations (term normal newborns, small and large for gestational age infants, and premature neonates) in a variety of nutritional states (fasting versus feeding, enteral versus parenteral) using a constant tracer infusion of 1-13C leucine combined with respiratory calorimetry. In addition, similar techniques (using 1-14C leucine) will be employed to quantitate leucine metabolism in the normal and experimental intrauterine growth retarded fetal and neonatal lamb. The performance of these studies will 1) generate new information concerning leucine metabolism in several important neonatal populations, 2) determine how alterations in the intrauterine environment might affect fetal and neonatal leucine (and protein) metabolism, 3) further the understanding of protein accretion in the neonatal period, and 4) provide an objective means to assess different nutritional strategies.